Basically your relative is inviting someone to share their house. I can't see how they could rent out just "a room" with no bathroom, or kitchen access etc. If there are two bathrooms in the house, one could be assigned to the lodger, but if there is only one, the relative would have to share, perhaps arranging specific times for use of kitchen, laundry facilities and bathroom.
Will the lodger keep their own room clean, or will the relative be providing cleaning services?
If the house-owner would prefer not to share the kitchen, is there space in the lodger's room to set up a mini-kitchen of kettle, toaster, fridge and microwave?
Will the current wifi system be sufficient for the lodger to work from home, if necessary?
Is extra furniture such as bed, wardrobe, desk and chair for working, going to be needed, or will the lodger be bringing some of their own?
House insurance will increase.
Work out beforehand what the extra heating, laundry, cooking, lighting and rubbish costs will be, recognising that costs will be much higher in winter.
Include all these extra costs in the rental, look at local advertisements, and see how much actual benefit there will be for your relative.